A study of the effects of mutations in DNA polymerase I in Escherichia coli and Salmonella typhimurium is being carried out, both genetic and enzymological aspects are being stressed. The effects of known mutations on the structure and enzymatic function of the DNA polymerase molecule is under investigation. A series of DNA polymerase I mutants are being selected in S. typhimurium for the purpose of examining their effects on plasmid replication.